1. Field of the Invention
The present invention relates to laminated LC filters, and specifically relates to a laminated LC filter that has a wide pass band and an attenuation pole with sufficient attenuation near the pass band.
2. Description of the Related Art
As an LC filter that is suitable to achieve miniaturization and weight-saving and has excellent characteristics, a laminated LC filter is widely used in which LC parallel resonators defined by inductors and capacitors are configured in a multistage configuration inside a multilayer body where a plurality of dielectric layers are laminated; the inductors and capacitors of the LC parallel resonators are provided in the multilayer body using line electrodes, capacitor electrodes, ground electrodes, and via electrodes.
Laminated LC filters are required to have optimum frequency characteristics in accordance with desired usage thereof.
As such a laminated LC filter, a laminated LC filter (laminated band pass filter) is disclosed in International Publication No. WO 2012/077498.
FIG. 7 illustrates a laminated LC filter 1100 disclosed in International Publication No. WO 2012/077498. FIG. 7 is an exploded perspective view of the laminated LC filter 1100. FIG. 8 is an equivalent circuit diagram of the laminated LC filter 1100.
The laminated LC filter 1100 includes a multilayer body 101 in which 15 layers of dielectric layers 101a through 101o are laminated in sequence from the bottom to the top.
Input-output terminals (input terminals) 102a and 102b are respectively provided on both end surfaces of the dielectric layer 101a. Further, capacitor electrodes 103a and 103b and a ground electrode (earth electrode) 104a are provided on an upper-side main surface of the dielectric layer 101a. The capacitor electrode 103a is connected to the input-output terminal 102a and the capacitor electrode 103b is connected to the input-output terminal 102b. 
Although not shown in FIG. 7, one end of each of the input-output terminals 102a and 102b extends to a lower-side main surface of the dielectric layer 101a. 
The input-output terminals 102a and 102b are also provided on both end surfaces of each of the dielectric layers 101b through 101o to be explained later. However, to simplify the drawings and the explanation thereof, assignment of reference signs in the drawings and description in the specification are omitted in some cases.
Capacitor electrodes 103c and 103d are provided on an upper-side main surface of the dielectric layer 101b. Further, via electrodes 105a and 105b extend through both of the main surfaces of the dielectric layer 101b. The capacitor electrode 103c is connected to the input-output terminal 102a and the capacitor electrode 103d is connected to the input-output terminal 102b. The via electrodes 105a and 105b are both connected to the ground electrode 104a. 
A ground electrode 104b is provided on an upper-side main surface of the dielectric layer 101c. In addition, via electrodes 105c and 105d extend through both of the main surfaces of the dielectric layer 101c. The ground electrode 104b is connected to the via electrodes 105c and 105d. Further, the via electrode 105c is connected to the via electrode 105a and the via electrode 105d is connected to the via electrode 105b. 
Capacitor electrodes 103e and 103f are provided on an upper-side main surface of the dielectric layer 101d. In addition, six via electrodes 105e through 105j extend through both of the main surfaces of the dielectric layer 101d. The capacitor electrode 103e is connected to the input-output terminal 102a and the capacitor electrode 103f is connected to the input-output terminal 102b. The six via electrodes 105e through 105j are all connected to the ground electrode 104b. 
A capacitor electrode 103g is provided on an upper-side main surface of the dielectric layer 101e. In addition, six via electrodes 105k through 105p extend through both of the main surfaces of the dielectric layer 101e. The via electrode 105k is connected to the via electrode 105e, the via electrode 105l is connected to the via electrode 105f, the via electrode 105m is connected to the via electrode 105g, the via electrode 105n is connected to the via electrode 105h, the via electrode 105o is connected to the via electrode 105i, and the via electrode 105p is connected to the via electrode 105j. 
Capacitor electrodes 103h and 103i are provided on an upper-side main surface of the dielectric layer 101f. In addition, six via electrodes 105q through 105v extend through both of the main surfaces of the dielectric layer 101f. The capacitor electrode 103h is connected to the input-output terminal 102a and the capacitor electrode 103i is connected to the input-output terminal 102b. Further, the via electrode 105q is connected to the via electrode 105k, the via electrode 105r is connected to the via electrode 105l, the via electrode 105s is connected to the via electrode 105m, the via electrode 105t is connected to the via electrode 105n, the via electrode 105u is connected to the via electrode 105o, and the via electrode 105v is connected to the via electrode 105p. 
A ground electrode 104c is provided on an upper-side main surface of the dielectric layer 101g. In addition, six via electrodes 105w through 105ab extend through both of the main surfaces of the dielectric layer 101g. The ground electrode 104c is connected to the six via electrodes 105w through 105ab. Further, the via electrode 105w is connected to the via electrode 105q, the via electrode 105x is connected to the via electrode 105r, the via electrode 105y is connected to the via electrode 105s, the via electrode 105z is connected to the via electrode 105t, the via electrode 105aa is connected to the via electrode 105u, and the via electrode 105ab is connected to the via electrode 105v. 
In this specification, when reference signs are assigned to certain constituent elements, where the number of the stated constituent elements is no more than 26, alphabetical letters “a” through “z” are used; where the number thereof exceeds 26, combinations of an alphabetical letter “a” and alphabetical letters “a” through “z” are used; where the number thereof further exceeds another 26, combinations of an alphabetical letter “b” and alphabetical letters “a” through “z” are used. For example, a total of 69 via electrodes are provided in the laminated LC filter 1100, and these via electrodes are represented by using reference signs 105a through 105z, 105aa through 105az, and 105ba through 105bu, respectively.
Capacitor electrodes 103j and 103k are provided on an upper-side main surface of the dielectric layer 101h. Further, five via electrodes 105ac through 105ag extend through both of the main surfaces of the dielectric layer 101h. The five via electrodes 105ac through 105ag are all connected to the ground electrode 104c. 
Capacitor electrodes 103l and 103m are provided on an upper-side main surface of the dielectric layer 101i. In addition, seven via electrodes 105ah through 105an extend through both of the main surfaces of the dielectric layer 101i. The capacitor electrode 103l is connected to the via electrode 105aj and the capacitor electrode 103m is connected to the via electrode 105al. Further, the via electrode 105ah is connected to the capacitor electrode 103j, the via electrode 105ai is connected to the via electrode 105ac, the via electrode 105aj is connected to the via electrode 105ad, the via electrode 105ak is connected to the via electrode 105ae, the via electrode 105al is connected to the via electrode 105af, the via electrode 105am is connected to the via electrode 105ag, and the via electrode 105an is connected to the capacitor electrode 103k. 
Capacitor electrodes 103n and 103o are provided on an upper-side main surface of the dielectric layer 101j. In addition, five via electrodes 105ao through 105as extend through both of the main surfaces of the dielectric layer 101j. The capacitor electrode 103n is connected to the via electrode 105ao and the capacitor electrode 103o is connected to the via electrode 105as. Further, the via electrode 105ao is connected to the via electrode 105ah, the via electrode 105ap is connected to the via electrode 105ai, the via electrode 105aq is connected to the via electrode 105ak, the via electrode 105ar is connected to the via electrode 105am, and the via electrode 105as is connected to the via electrode 105an. 
Line electrodes 106a and 106b are provided on an upper-side main surface of the dielectric layer 101k. Further, five via electrodes 105at through 105ax extend through both of the main surfaces of the dielectric layer 101k. One end of the line electrode 106a is connected to the input-output terminal 102a and one end of the line electrode 106b is connected to the input-output terminal 102b. The via electrode 105at is connected to the capacitor electrode 103n, the via electrode 105au is connected to the via electrode 105ap, the via electrode 105av is connected to the via electrode 105aq, the via electrode 105aw is connected to the via electrode 105ar, and the via electrode 105ax is connected to the capacitor electrode 103o. 
Line electrodes 106c and 106d are provided on an upper-side main surface of the dielectric layer 1011. Further, seven via electrodes 105ay through 105be extend through both of the main surfaces of the dielectric layer 101l. One end of the line electrode 106c and one end of the line electrode 106d are both connected to the via electrode 105bb. The via electrode 105ay is connected to the other end of the line electrode 106a, the via electrode 105az is connected to the via electrode 105at, the via electrode 105ba is connected to the via electrode 105au, the via electrode 105bb is connected to the via electrode 105av, the via electrode 105bc is connected to the via electrode 105aw, the via electrode 105bd is connected to the via electrode 105ax, and the via electrode 105be is connected to the other end of the line electrode 106b. 
Four line electrodes 106e through 106h are provided on an upper-side main surface of the dielectric layer 101m. In addition, eight via electrodes 105bf through 105bm extend through both of the main surfaces of the dielectric layer 101m. One end of the line electrode 106e is connected to the via electrode 105bf, the other end of the line electrode 106e is connected to the via electrode 105bj, one end of the line electrode 106f is connected to the via electrode 105bg, the other end of the line electrode 106f is connected to the via electrode 105bi, one end of the line electrode 106g is connected to the via electrode 105bj, the other end of the line electrode 106g is connected to the via electrode 105b1, one end of the line electrode 106h is connected to the via electrode 105bk, and the other end of the line electrode 106h is connected to the via electrode 105bm. Further, the via electrode 105bf is connected to the via electrode 105ay, the via electrode 105bg is connected to the via electrode 105az, the via electrode 105bh is connected to the via electrode 105ba, the via electrode 105bi is connected to the other end of the line electrode 106c, the via electrode 105bj is connected to the other end of the line electrode 106d, the via electrode 105bk is connected to the via electrode 105bc, the via electrode 105b1 is connected to the via electrode 105bd, and the via electrode 105bm is connected to the via electrode 105be. 
Four line electrodes 106i through 106l are provided on an upper-side main surface of the dielectric layer 101n. In addition, eight via electrodes 105bn through 105bu extend through both of the main surfaces of the dielectric layer 101n. One end of the line electrode 106i is connected to the via electrode 105bn, the other end of the line electrode 106i is connected to the via electrode 105bp, one end of the line electrode 106j is connected to the via electrode 105bo, the other end of the line electrode 106j is connected to the via electrode 105bq, one end of the line electrode 106k is connected to the via electrode 105br, the other end of the line electrode 106k is connected to the via electrode 105bt, one end of the line electrode 106l is connected to the via electrode 105bs, and the other end of the line electrode 106l is connected to the via electrode 105bu. Further, the via electrode 105bn is connected to the one end of the line electrode 106e, the via electrode 105bo is connected to the one end of the line electrode 106f, the via electrode 105bp is connected to the other end of the line electrode 106e, the via electrode 105bq is connected to the other end of the line electrode 106f, the via electrode 105br is connected to the one end of the line electrode 106g, the via electrode 105bs is connected to the one end of the line electrode 106l, the via electrode 105bt is connected to the other end of the line electrode 106g, and the via electrode 105bu is connected to the other end of the line electrode 106h. 
The input-output terminals 102a and 102b are provided on both end surfaces of the dielectric layer 101o. One end of each of the input-output terminals 102a and 102b extends to an upper-side main surface of the dielectric layer 101o. 
As discussed above, in the laminated LC filter 1100, the input-output terminals 102a and 102b are provided on the surface of the multilayer body 101. Inside the multilayer body 101, the capacitor electrodes 103a through 103o, the ground electrodes 104a through 104c, the via electrodes 105a through 105bu, and the line electrodes 106a through 106l are provided.
The laminated LC filter 1100 disclosed in International Publication No. WO 2012/077498 has the above-discussed structure and has an equivalent circuit as shown in FIG. 8.
The laminated LC filter 1100 is configured such that four LC parallel resonators Re1 through Re4 are inserted between the ground and a signal line connecting the input-output terminals 102a and 102b. 
An inductor L1 and a capacitor C1 are connected in parallel in the first-stage LC parallel resonator Re1.
An inductor L2 and a capacitor C2 are connected in parallel in the second-stage LC parallel resonator Re2.
An inductor L3 and a capacitor C3 are connected in parallel in the third-stage LC parallel resonator Re3.
An inductor L4 and a capacitor C4 are connected in parallel in the fourth-stage LC parallel resonator Re4.
Note that the inductor L2 in the second-stage LC parallel resonator Re2 and the inductor L3 in the third-stage LC parallel resonator Re3 are connected to each other and then connected to the ground through a common inductor L23.
A capacitor C14 is connected, in parallel to the signal line, between the input-output terminals 102a and 102b. 
With reference to FIGS. 7 and 8, a relationship between the structure and the equivalent circuit of the laminated LC filter 1100 will be described next.
In a laminated LC filter, to improve a Q value, multi-layered line electrodes having a plurality of layers are provided in some cases. As such, in the laminated LC filter 1100, the one end of the line electrode 106e and the one end of the line electrode 106i are connected through the via electrode 105bn while the other ends thereof are connected through the via electrode 105bp, such that the line electrodes 106e and 106i are multi-layered. Similarly, the one end of the line electrode 106f and the one end of the line electrode 106j are connected with the via electrode 105bo while the other ends thereof are connected through the via electrode 105bq, such that the line electrodes 106f and 106j are multi-layered. Further, the one end of the line electrode 106g and the one end of the line electrode 106k are connected through the via electrode 105br while the other ends thereof are connected through the via electrode 105bt, such that the line electrodes 106g and 106k are multi-layered. Furthermore, the one end of the line electrode 106h and the one end of the line electrode 106l are connected through the via electrode 105bs while the other ends thereof are connected through the via electrode 105bu, such that the line electrodes 106h and 106l are multi-layered.
The inductor L1 in the first-stage LC parallel resonator Re1 is defined by a loop connecting the input-output terminal 102a, the line electrode 106a, the via electrode 105ay, the via electrode 105bf, the line electrode 106e and the line electrode 106i that are connected to one another through the via electrode 105bn and the via electrode 105bp, the via electrode 105bh, the via electrode 105ba, the via electrode 105au, the via electrode 105ap, the via electrode 105ai, the via electrode 105ac, and the ground electrode 104c. 
The capacitor C1 in the first-stage LC parallel resonator Re1 is primarily defined by a capacitance produced between the ground electrode 104b and the capacitor electrodes 103c and 103e, and a capacitance produced between the capacitor electrode 103h and the ground electrode 104c. The capacitor electrodes 103c, 103e, and 103h are all connected to the input-output terminal 102a. 
The inductor L1 and the capacitor C1 in the first-stage LC parallel resonator Re1 are not directly connected to one another, but are indirectly connected through the input-output terminal 102a. 
The inductor L2 in the second-stage LC parallel resonator Re2 is defined by a loop connecting the capacitor electrode 103j and capacitor electrode 103n connected to each other through the via electrode 105ah and via electrode 105ao, the via electrode 105at, the via electrode 105az, the via electrode 105bg, the line electrode 106e and the line electrode 106i connected to one another through the via electrode 105bo and the via electrode 105bq, the via electrode 105bi, and the line electrode 106c. 
The capacitor C2 in the second-stage LC parallel resonator Re2 is primarily defined by a capacitance produced between the capacitor electrodes 103j and 103n, and the ground electrode 104c and the capacitor electrode 103l. Note that the capacitor electrodes 103j and 103n are connected through the via electrodes 105ah and 105ao as discussed above. Further, the capacitor electrode 103l is connected to the ground electrode 104c through the via electrodes 105aj and 105ad. 
The inductor L3 in the third-stage LC parallel resonator Re3 is defined by a loop connecting the capacitor electrode 103k and capacitor electrode 103o connected to each other through the via electrode 105an and via electrode 105as, the via electrode 105ax, the via electrode 105bd, the via electrode 105b1, the line electrode 106g and line electrode 106k are connected to one another through the via electrode 105br and via electrode 105bt, the via electrode 105bj, and the line electrode 106d. 
The capacitor C3 in the third-stage LC parallel resonator Re3 is primarily defined by a capacitance produced between the capacitor electrodes 103k and 103o, and the ground electrode 104c and the capacitor electrode 103m. Note that the capacitor electrodes 103k and 103o are connected through the via electrodes 105an and 105as as discussed above. Further, the capacitor electrode 103m is connected to the ground electrode 104c through the via electrodes 105al and 105af. 
The inductor L4 in the fourth-stage LC parallel resonator Re4 is defined by a loop connecting the input-output terminal 102b, the line electrode 106b, the via electrode 105be, the via electrode 105bm, the line electrode 106h and line electrode 106l connected to each other through the via electrode 105bs and via electrode 105bu, the via electrode 105bk, the via electrode 105bc, the via electrode 105aw, the via electrode 105ar, the via electrode 105am, the via electrode 105ag, and the ground electrode 104c. 
The capacitor C4 in the fourth-stage LC parallel resonator Re4 is primarily defined by a capacitance produced between the ground electrode 104b and the capacitor electrodes 103d and 103f, and a capacitance produced between the capacitor electrode 103i and the ground electrode 104c. Note that the capacitor electrodes 103d, 103f, and 103i are all connected to the input-output terminal 102a. 
The inductor L4 and the capacitor C4 in the fourth-stage LC parallel resonator Re4 are not directly connected, but are indirectly connected through the input-output terminal 102b. 
As discussed above, the inductor L2 in the second-stage LC parallel resonator Re2 and the inductor L3 in the third-stage LC parallel resonator Re3 are connected to each other and then connected to the ground through the common inductor L23. The common inductor L23 is defined by a path connecting the via electrodes 105bb, 105av, 105aq, 105ak, and 105ae, and is connected to the ground electrode 104c. Note that the common inductor L23 can be considered to be a portion of the inductor L2 in the second-stage LC parallel resonator Re2 and also a portion of the inductor L3 in the third-stage LC parallel resonator Re3.
The capacitor C14 is primarily defined by a capacitance produced by the capacitor electrodes 103e and 103h, the capacitor electrode 103g as a floating electrode, and the capacitor electrodes 103f and 103i. 
In the laminated LC filter 1100, the loop of the inductor L1 in the first-stage LC parallel resonator Re1 and the loop of the inductor L2 in the second-stage LC parallel resonator Re2 are disposed in parallel, and the winding directions thereof are the same. As such, the inductor L1 and the inductor L2 are coupled by magnetic coupling M12.
Similarly, the loop of the inductor L3 in the third-stage LC parallel resonator Re3 and the loop of the inductor L4 in the fourth-stage LC parallel resonator Re4 are disposed in parallel, and the winding directions thereof are the same. As such, the inductor L3 and the inductor L4 are coupled by magnetic coupling M34.
The loop of the inductor L2 in the second-stage LC parallel resonator Re2 and the loop of the inductor L3 in the third-stage LC parallel resonator Re3 are parallel to each other, but are obliquely disposed and the winding directions thereof are different from each other. Therefore, the strength of magnetic coupling therebetween is weak. As such, in the laminated LC filter 1100, the inductor L2 and the inductor L3 are connected to each other and then connected to the ground through the common inductor L23, so as to obtain magnetic coupling M23 between the inductor L2 and the inductor L3.
Detailed description about this will be provided below. That is, in the laminated LC filter 1100, an attenuation pole is provided near the pass band by making the winding direction of the loop of the inductor L2 and the winding direction of the loop of the inductor L3 to differ from each other, so as to make it possible to obtain high attenuation characteristics. However, when the winding direction of the loop of the inductor L2 and the winding direction of the loop of the inductor L3 are different, the strength of magnetic coupling between the inductor L2 and the inductor L3 is weak and, consequently, the pass band is narrowed. As such, in the laminated LC filter 1100, as discussed above, the inductors L2 and L3 are connected to each other and then connected to the ground through the common inductor L23, such that the magnetic coupling M23 between the inductors L2 and L3 is strengthened and the pass band is widened.
The first-stage LC parallel resonator Re1 and the fourth-stage LC parallel resonator Re4 are capacitively coupled through the capacitor C14, that is, coupled to each other while bypassing the other LC parallel resonators.
FIG. 9 illustrates frequency characteristics of the laminated LC filter 1100 disclosed in International Publication No. WO 2012/077498.
In the laminated LC filter 1100 disclosed in International Publication No. WO 2012/077498, the inductors L2 and L3 are connected to each other and then connected to the ground through the common inductor L23 as shown in FIG. 8, such that the magnetic coupling M23 between the inductors L2 and L3 is strengthened and the pass band is widened as shown in FIG. 9.
However, in the laminated LC filter 1100, because the inductors L2 and L3 are connected to each other and then connected to the ground through the common inductor L23, attenuation of attenuation poles near the pass band, especially attenuation of the attenuation pole on a higher frequency side of the pass band is insufficient, as shown in FIG. 9, so that high attenuation characteristics cannot be obtained.